By means of generating and measuring electric alternating fields an approach of an object towards a sensor zone is measured in capacitive sensor systems. Derived from the measurement signal may be functions, for example switching functions of an electric device, in particular of an electric handheld device. This implies that the sensor signal, for activating a certain function, exceeds a predetermined switching threshold during the approach towards the sensor electrodes of the capacitive sensor system and at contacting the sensor electrodes, respectively.
Known in prior art are sensor systems and capacitive sensor systems, respectively, which work using predetermined and fixedly predefined, respectively, switching thresholds. The provision of fixed switching thresholds, however, has the disadvantage that each capacitive Sensor has a different sensibility and sensitivity, respectively, for example because variations in the production process result in that the switching threshold in each sensor is exceeded with a different amount of approach and with differently strong contact, respectively.
Furthermore, also the ambient conditions of an electric handheld device, in which a capacitive sensor is provided, may result in that the sensitivity of the capacitive sensor system depends on changing ambient conditions. For example, a grasping of an electric handheld device by a hand may result in that a switching threshold provided for the activation of the electric handheld device is exceeded, while during a grasping of a handheld device by a hand comprising a glove it may result in that the respective switching threshold is not exceeded. This has negative effects on the handling of the electric handheld device, because the handheld device during grasping with a hand without a glove reacts differently as compared to a grasping of the handheld device by a hand comprising a glove.
It is therefore desirable to provide a capacitive sensor system, which always has a substantially unchanging sensibility and sensitivity, respectively, independent of the external influences and ambient conditions, respectively.
In order to solve this problem it is known in conventional systems to adjust the switching threshold of a capacitive sensor system during an initialization phase, for example during switching on a handheld device, to such adjust the switching threshold to the ambient conditions existing during the initialization phase. However, in this case it is disadvantageous that the switching threshold only is set once, namely during switching on the handheld device. Admittedly this has the advantage that the switching threshold may be different after several initialization phases. On the other hand, this method, however, has the significant disadvantage that the switching threshold set during the initialization phase remains unchanged during the entire operation of the device so that the problems mentioned above only are avoided in part.